1. Field of the Invention
The present invention relates to a hydraulic shock absorber.
2. Description of the Related Art
A conventional hydraulic shock absorber described, for example, in Japanese Unexamined Patent Publication No. 2007-177877 (patent document 1) is structured, as shown in FIG. 9, such that a piston rod 2 attached to one of a vehicle body side and an axle side is inserted into an oil chamber of a cylinder 1 attached to the other of the vehicle body side and the axle side, the oil chamber of the cylinder 1 is comparted into a piston side oil chamber 4A and a rod side oil chamber 4B by the piston 3 provided in a leading end portion of the piston rod 2, and a damping force generating device 5 is provided on the piston 3. Further, an oil reservoir chamber 6 compensating a volumetric capacity (including a volumetric capacity corresponding to a temperature extension of the oil) of the piston rod 2 moving forward and backward with respect to the oil chambers 4A and 4B of the cylinder 1 communicates with the piston side oil chamber 4A of the cylinder 1, and a valve housing 7 provided with a damping force generating device 8 is interposed between the piston side oil chamber 4A of the cylinder 1 and the oil reservoir chamber 6. The oil reservoir chamber 6 is pressurized by an air chamber 6A (a bladder, a free piston or the like may be interposed between the oil reservoir chamber 6 and the air chamber 6A).
In this case, the damping force generating device 5 includes a compression side damping valve 5A opening and closing a compression side flow path 3A provided in the piston 3, and an extension side damping valve 5B opening and closing an extension side flow path 3B provided in the piston 3. The damping force generating device 8 includes a compression side damping valve 8A opening and closing a compression side flow path 7A provided in the valve housing 7, and an extension side damping valve 8B opening and closing an extension side flow path 7B provided in the valve housing 7.
In a compression stroke, the pressure of the oil in the piston side oil chamber 4A rises and the oil flows out through the compression side damping valve 8A of the compression side flow path 7A to the oil reservoir chamber 6 and through the compression side damping valve 5A of the compression side flow path 3A to the rod side oil chamber 4B, thereby generating a compression side damping force based on a flow path resistance of the compression side damping valve 8A and the compression side damping valve 5A. As a result, an amount of oil corresponding to the volumetric capacity of the forward movement of the piston rod 2 is discharged to the oil reservoir chamber 6 through the compression side damping valve 8A. In an extension stroke, the pressure of the oil in the rod side oil chamber 4B rises and the oil flows out through the extension side damping valve 5B of the extension side flow path 3B to the piston side oil chamber 4A, thereby generating an extension side damping force based on a flow path resistance of the extension side damping valve 5B, and an amount of oil corresponding to the volumetric capacity of the backward movement of the piston rod 2 is replenished to the piston side oil chamber 4A from the oil reservoir chamber 6 through the extension side damping valve 8B of the extension side flow path 7B.
In the conventional hydraulic shock absorber, since the oil with increased pressure in the piston side oil chamber 4A (the COMP chamber) flows separately into two flow paths 7A and 3A respectively directed to the oil reservoir chamber 6 and the rod side oil chamber 4B (the TEN chamber) in the compression stroke as shown in FIG. 9, the pressure of the rod side oil chamber 4B is changed from a positive pressure to a negative pressure based on a balance of the flow path resistance between the compression side damping valves 8A and 5A of the flow paths 7A and 3A. In other words, the pressure of the rod side oil chamber 4B is changed from the positive pressure to the negative pressure based on the balance of the flow path resistance of the compression side damping valve 8A, the air pressure of the air chamber 6A, and the flow path resistance of the compression side damping valve 5A, and if the flow path resistance of the compression side damping valve 5A is too large, a cavitation is generated in the rod side oil chamber 4B and a pause in a damping force occurs when reversing to the extension stroke.
In the extension stroke, the oil with increased pressure in the rod side oil chamber 4B only flows out through one flow path 3B to the piston side oil chamber 4A, and the pressure in the piston side oil chamber 4A does not generate any fluctuation while depending only upon the air pressure of the air chamber 6A.